Two important parameters for evaluating production of an underlying oil or gas bearing formation are to determine the permeability and porosity of core samples taken from the formation. A measurement of permeability of the core provides an indication as to how fast the oil or gas will flow from the formation upon production whereas a measurement porosity provides information as to the amount of oil or gas contained within the formation. In conducting such tests on core samples, especially when overburden pressures are applied to the core samples, perforated end plug plates are used on each end of the sample to aid in the distribution of gas into the sample.
The determination of both porosity and permeability are based upon complex mathematical determinations and both are common measurements in the oil and gas industry. An understanding of these mathematical formulas is not necessary for the understanding of the present invention. However, a discussion of the mathematical formulas for determining Klinkenberg permeability, the Klinkenberg slip factor and the Forcheimer turbulence factor observed in core plugs is set forth in the inventor's prior publication entitled "A Rapid Accurate Unsteady-State Klinkenberg Permeameter", Society of Petroleum Engineers Journal, October 1972, pages 383-397. In that publication, a method and apparatus for performing permeability tests on core samples is set forth. In that disclosure, each sample core is manually loaded into a Hassler core holder and the sleeve contained therein is then pressurized to simulate an overburden pressure. A gas, such as nitrogen, is then introduced through an end plate having circular formed holes into one end of the core and the passage of the gas through the core into a second end plate is then determined to ascertain the permeability. End plug plates are utilized at opposing ends of the core sample to aid in the distribution of gas and to provide structural support to the core sample.
In addition, prior to the filing of this invention, a patentability search was conducted on the above identified related application which uncovered the following patents:
______________________________________ Inventor Reg. No. Reg. Date ______________________________________ Bowman 966,078 Aug. 2, 1910 Dietert et al. 2,516,188 July 25, 1950 Reichertz 2,539,355 Jan. 23, 1951 Leas 2,618,151 Nov. 18, 1952 Herzog et al. 2,737,804 Mar. 13, 1956 Dotson 2,745,057 May 8, 1956 Donaldson 3,158,020 Nov. 24, 1964 Heuer, Jr. et al. 3,199,341 Aug. 10, 1965 McMillen 3,839,899 Oct. 8, 1974 Wilkins 4,043,407 Aug. 23, 1977 Turner et al. 4,083,228 Apr. 11, 1978 Neri 4,227,397 Oct. 14, 1980 Wiley 4,253,327 Mar. 3, 1981 Heitmann et al. 4,287,754 Sept. 8, 1981 Pezzi 4,403,501 Sept. 13, 1983 Hains 4,430,890 Feb. 14, 1984 Holt 4,454,095 June 12, 1984 ______________________________________
Only the following disussed patents disclosed types of perforated end plugs.
The Wiley patent sets forth a method and apparatus for measuring core permeability at overburden conditions of both pressure and temperature. Each core must be manually loaded into a sleeve having end plugs inserted into the sleeve. Then the entire assembly is placed into a hydrostatic cell wherein hydraulic fluid is pressurized around the end plugs and the sleeve to simulate the overburden pressure. The fluid is then injected through one end plug, through a sintered plate, through the core, out a second sintered plate and through the opposing end plug.
In Leas, a manually loaded cell for measuring relative permeability is disclosed wherein a flexible elastic sleeve selectively pressurizes the sides of the core during testing so as to simulate overburden stress. Fluids are injected into the end of the core to measure the permeability of the core. To insert or remove the core, a vacuum is pulled around the elastic sleeve so that the core can be manually removed or inserted. Porous disks are placed on each end of the core to aid in the distribution of the fluid to and from the core. The porous disks of Leas have two embodiments. The first embodiment has a rectangular grid of channels on the side of the plate abutting the core sample and the second embodiment provides a shallow cylindrical cavity. The cavity and grooves both are in fluid communication with a center hole. The opposite side of each plate is flat.
Heuer, Jr. et al. discloses a method and apparatus for measuring compressibility of core samples by encapsulating the core sample in a fluid-impervious sheet such as flexible plastic and then suspending the core sample in a pressure vessel and subjecting the sample to high pressure while passing fluids to and from opposing ends of the core sample through fluid-permeable steel disks.
The Morgan patent sets forth a method of sealing cores while determining the permeability of the core by providing a counter-pressure environment around the core with an atmosphere of non-wetting fluid. The pressure eliminates the use of sealing material such as pitch, tar, or a separate sealing medium such as plastic or rubber. A capillary diaphragm is used on opposing ends of the core sample.
A disadvantage with prior art approaches as found in the Leas "waffle" design occurs when the applied stresses cause the plate to deform (imprint) the ends of the core. This not only may damage the core, but also blocks the gas passageways in the plate possibly effectuating a less than uniform distribution of gas in the core. Non-uniform distribution of gas may cause errors to occur in the permeability or porosity readings.
None of the above discussed patents set forth an end plug design of the present invention which includes the plug, having an array of fluid passage channels formed thereon, and a two-sided porous disk, one side abutting the fluid passage channels and the other side abutting the core.